The overall objective is to establish an in vitro system to permit detailed study of fluoride, particularly at low levels, on developing mammalian tooth germs. Information will be sought at the light microscope and ultrastructural levels of fluoride on ameloblasts, odontoblasts, including developmental stages. Cellular effects will be investigated in relation to effects on early dentinal and enamel matrix secretion as a response to fluoride. A primary aim is to investigate why ameloblasts are exquisitely sensitive to fluoride at doses as low as 1 ppm when considerably higher doses are required for observable effects in bone or other tissues. The advantage of this system is that it will permit study of fluoride at very low known doses in stages before mineralization, heretofore impossible in animals. Principal objectives are to determine: (1) Observable effects at fluoride levels ranging from 100 to 2.0 ppm in the entire molar tooth germ. (2) The minimal duration required for fluoride activity at a range of fluoride levels. (3) The reversibility of effects caused by fluoride at a range of dose levels. (4) Effects of fluoride on rates of cell division in tooth germs. (5) Whether fluoride has direct effects on both the enamel and dentin forming cells or whether direct effect on either may have secondary effects on the other. (6) Whether fluoride affects the ability of ameloblasts and ondotoblasts at appropriate dose levels to incorporate and utilize matrix precursor substances. (7) The nature of vacuoles and vesicles already observed, in the pilot study, at low doses with arrest of enamel matrix secretion. One-day-old mouse first mandibular molars will be grown in organ culture in liquid medium containing fluoride. Specimens will be prepared for observation by light and electron microscopy and for autoradiography.